A humidity sensor for detecting humidity in atmosphere is disclosed in, for example, JP-A-H02-140654. This sensor includes a humidity sensitive film made of high-polymer material having moisture-absorption characteristics. The humidity sensitive film has multiple fine pores, dimensions of which are about a few nanometers. The moisture is absorbed in the fine pores, and desorbed from the fine pores. In accordance with absorption and desorption of the moisture, dielectric constant of the humidity sensitive film is changed. In general, the dielectric constant of the moisture is larger than that of the high-polymer material. Accordingly, when the humidity sensitive film absorbs the moisture, the dielectric constant of the film becomes larger. By detecting a capacitance of the humidity sensitive film, the humidity in atmosphere is detected.
FIGS. 6A to 6D shows sensor characteristics of the humidity sensor with reference to maximum amount of moisture in the humidity sensitive film. Here, the maximum amount of moisture is defined such that the humidity sensitive film F is capable of absorbing the moisture equal to or smaller than the maximum amount of moisture. FIGS. 6A to 6C are cross sectional views explaining change of maximum amount of moisture in the humidity sensitive film in accordance with humidity change. FIG. 6D shows sensor characteristics of the humidity sensor in each humidity condition. Here, the sensor characteristics relate to a relationship between a sensor output and a relative humidity in atmosphere. In this sensor, when the sensor is manufactured, the sensor characteristics are set to be a graph DB in FIG. 6B. FIG. 6A shows the sensor when the humidity is comparatively low, and FIG. 6C shows the sensor when the humidity is comparatively high. When the humidity becomes higher, the film F swells, and therefore, the sensor changes from a state in FIG. 6B to a state in FIG. 6C. When the humidity becomes lower, the film F shrinks, and therefore, the sensor changes from a state in FIG. 6B to a state in FIG. 6A.
When the sensor is just manufactured, the diameter of the fine pore P in the humidity sensitive film F is defined as DB, and the maximum amount of moisture in the film F is defined as VB. When the sensor is maintained in a comparatively high humidity condition, the film F swells in accordance with absorption of moisture. In this case, the diameter of the fine pore P changes from DB to DC. Thus, the average of diameter distribution of the fine pores P becomes larger. In this case, the maximum amount of moisture in the film F increases from VB to VC. Thus, the moisture in the atmosphere is easily absorbed in the film F. As a result, even when the humidity is the same, the capacitance change of the film F becomes larger. Thus, as shown in FIG. 6D, the sensor output becomes larger, so that the sensor characteristics changes from DB to DC. Specifically, the sensitivity of the sensor with reference to the humidity becomes larger.
In this case, the following problems occur. For example, when the humidity in the atmosphere is RH2 in FIG. 6D, the sensor output is V2 in a case where the sensor has the sensor characteristics DB, which is the initial sensor characteristics. However, the maximum amount of moisture in the film F increases, so that the sensor characteristics of the sensor change to the characteristics DC. Thus, the output of the sensor changes from V2 to V3. In this case, since the sensor characteristics are set to be DB, the humidity sensor calculates the humidity as RH3, which corresponds to the output V3. Although the actual humidity is RH2, the detected humidity is RH3, which is higher than the actual humidity.
When the sensor is maintained in a comparatively low humidity condition, as shown in FIG. 6A, the moisture is desorbed from the film F. In this case, the diameter of the fine pore P changes from DB to DA. Thus, the average of diameter distribution of the fine pores P becomes smaller. In this case, the maximum amount of moisture in the film F decreases from VB to VA. Thus, the moisture in the atmosphere is not easily absorbed in the film F. As a result, even when the humidity is the same, the capacitance change of the film F becomes smaller. Thus, as shown in FIG. 6D, the sensor output becomes smaller, so that the sensor characteristics changes from DB to DA. Specifically, the sensitivity of the sensor with reference to the humidity becomes smaller.
In this case, the following problems occur. For example, when the humidity in the atmosphere is RH2 in FIG. 6D, the sensor output is V2 in a case where the sensor has the sensor characteristics DB, which is the initial sensor characteristics. However, the maximum amount of moisture in the film F increases, so that the sensor characteristics of the sensor change to the characteristics DA. Thus, the output of the sensor changes from V2 to V1. In this case, since the sensor characteristics are set to be DB, the humidity sensor calculates the humidity as RH1, which corresponds to the output V1. Although the actual humidity is RH2, the detected humidity is RH1, which is lower than the actual humidity.
Thus, the sensor characteristics changes in accordance with environment of the sensor disposed before humidity detection. Accordingly, detection error may be observed.